[1] Tsinghua Univ, Dept Phys, Beijing 100084, Peoples R China
[2] Tsinghua Univ, Tsinghua Foxconn Nanotechnol Res Ctr, Beijing 100084, Peoples R China
[3] Peking Univ, Sch Phys, Electron Microscopy Lab, Beijing 100087, Peoples R China
[4] Peking Univ, Sch Phys, State Key Lab Mesoscop Phys, Beijing 100087, Peoples R China